Novel tennis ball

ABSTRACT

Tennis ball which comprises an elastomeric polyurethane foam having a ball shape and a density of 250-800 kg/m 3 , which foam is covered with a textile material.

The present invention relates to a novel tennis ball and to a processfor making it.

Currently tennis balls can be divided in two types : pressurized onesand pressureless ones. Pressurized balls are preferred since theyprovide better playing comfort (less vibrations and strain on the humanjoints). Pressurized balls however lose their pressure and hence theirproperties over time. Pressureless balls on the other hand hold theirproperties longer but do not provide the same comfort.

Tennis balls comprise a hollow inner rubber core covered with a textilematerial, normally a mixture of wool and nylon. The inner core isconstructed of two half-shell pieces of formed rubber which are joinedtogether with adhesive to form a single core. Two dumbbell shaped piecesof textile material are attached to the ball core by means of adhesiveto give the tennis ball its classic appearance. The thickness anddensity of the textile material is matched to the court type for whichthe ball is designed. Pressurized balls are made by filling the corewith air or other gas at a pressure above ambient pressure; pressurelessballs are made from a harder core. The inner core is generally made ofrubber containing additives, for pressurized as well as pressurelessballs.

It would be an advantage when the good properties of the pressurized andpressureless balls could be combined.

Surprisingly, we have found a new tennis ball which provides the samecomfort as the traditional pressurized ball, which maintains itsproperties at least as long as the pressureless one, which can be easilymade and which can have exactly the same outlook as the traditionalball.

Therefore the present invention relates to a tennis ball which comprisesan elastomeric polyurethane foam having a ball shape and a density of250-800 kg/m³ which foam is covered with a textile material and whichfoam has been prepared by reacting an aromatic polyisocyanate and apolyol comprising at least 30% by weight of a polyol having a level ofunsaturation of at most 0.03 meq/g and using a blowing agent, whereinthe polyol having a level of unsaturation of at most 0.03 meq/g isselected from polyester polyols, polyoxypropylene polyols,polyoxyethylene polyoxypropylene polyols, polyoxyethylene polyols,polyoxybutylene polyols, polyoxyalkylene polyols comprising oxybutylenegroups together with oxyethylene and/or oxypropylene groups,polyoxyalkylene polyester polyols and mixtures of such polyols with theproviso that the use of a polyoxyethylene polyoxypropylene polyol havingan oxyethylene content of 50-90% by weight, calculated on the weight ofthe polyol, as the only polyol having a level of unsaturation of at most0.03 meq/g is excluded.

The process for making a tennis ball according to the present inventioncomprises making a ball shaped elastomeric polyurethane foam by puttingan aromatic polyisocyanate, a polyol comprising at least 30% by weightof a polyol having a level of unsaturation of at most 0.03 meq/g and ablowing agent into a ball shaped mould and by allowing these ingredientsto form the polyurethane foam, removing the foam from the mould andcovering the foam with a textile material, wherein the polyol having alevel of unsaturation of at most 0.03 meq/g is selected from polyesterpolyols, polyoxypropylene polyols, polyoxyethylene polyoxypropylenepolyols, polyoxyethylene polyols, polyoxybutylene polyols,polyoxyalkylene polyols comprising oxybutylene groups together withoxyethylene and/or oxypropylene groups, polyoxyalkylene polyesterpolyols and mixtures of such polyols with the proviso that the use of apolyoxyethylene polyoxypropylene polyol having an oxyethylene content of50-90% by weight, calculated on the weight of the polyol, as the onlypolyol having a level of unsaturation of at most 0.03 meq/g is excluded.

The tennis ball preferably has the following further characteristics :

-   -   weight of the ball: 50-70 and preferably 55-60 g,    -   density of the ball: 230-540 kg/m³,    -   diameter of the ball: 6-8 cm,    -   rebound of the ball (after a free fall of 254 cm): 110-160 and        preferably 120-150 cm,    -   compression load deflection of the ball (CLD) of 0.4-0.8 cm        (forward) and 0.5-1.2 cm (return). CLD is measured, using the        Percey Herbert Stevens equipment as disclosed in GBP 230250 or        similar equipment approved by the International Tennis        Federation, at 8.2 kg load and over 3 axes with a variation of        at most 0.076 cm,    -   thickness of the textile material: 0.5-5 mm,    -   density of the textile material: 150-250 kg/m³.

Most preferably the tennis ball has all these characteristics incombination.

In order to improve the playing comfort of the balls according to thepresent invention even further it is advantageous to have a CLD (forwardand return) at the higher end of the above range; this is achieved bylowering the hardblock content when making the polyurethane foam.

The International Tennis Federation requires a rebound, a CLD forwardand a CLD return, amongst other requirements, within above ranges.Tennis balls according to the present invention may have physicalproperties within the ranges as required by the ITF.

General Procedure for Testing:

Before a ball is tested it shall be steadily compressed by approximatelyone inch (2.54 cm) on each of three diameters at right angles to oneanother in succession; this process to be carried out three times (ninecompressions in all). All tests are to be completed within two hours ofprecompression. Unless otherwise specified all tests shall be made at atemperature of approximately 68° Fahrenheit (20° Celsius), a relativehumidity of approximately 60% and, unless otherwise specified, anatmospheric pressure of approximately 30 inches Hg (102 kPa). All ballsshall be kept at this temperature and this humidity for 24 hours priorto testing and when testing is commenced.

The use of foams in tennis balls has been disclosed in the past;interesting art is U.S. Pat. No. 5,413,331, US 2005/014854, EP 1148085,KR 2001/002975, EP 10645, GB 2008954, NL 9201353, DE 3131705, GB2001538, GB 910701, EP 1344555, DE 2911430 and WO 03/41813.

WO 2008/000590, published after the priority date of the presentapplication, discloses tennis balls made from a polyoxyethylenepolyoxypropylene polyol having an oxyethylene content of 50-90% byweight.

However the presently claimed tennis balls have not been disclosed.

In the context of the present invention the following terms have thefollowing meaning:

-   -   1) isocyanate index or NCO index or index:        -   the ratio of NCO-groups over isocyanate-reactive hydrogen            atoms present in a formulation, Given as a percentage:

$\frac{\left\lbrack {N\; C\; O} \right\rbrack \times 100\; (\%)}{\left\lbrack {{active}\mspace{14mu} {hydrogen}} \right\rbrack}.$

-   -   -   In other words the NCO-index expresses the percentage of            isocyanate actually used in a formulation with respect to            the amount of isocyanate theoretically required for reacting            with the amount of isocyanate-reactive hydrogen used in a            formulation.        -   It should be observed that the isocyanate index as used            herein is considered from the point of view of the actual            polymerisation process preparing the elastomer involving the            isocyanate ingredient and the isocyanate-reactive            ingredients. Any isocyanate groups consumed in a preliminary            step to produce modified polyisocyanates (including such            isocyanate-derivatives referred to in the art as            prepolymers) or any active hydrogens consumed in a            preliminary step (e.g. reacted with isocyanate to produce            modified polyols) are not taken into account in the            calculation of the isocyanate index. Only the free            isocyanate groups and the free isocyanate-reactive hydrogens            (including those of the water) present at the actual            polymerisation stage are taken into account.

    -   2) The expression “isocyanate-reactive hydrogen atoms” as used        herein for the purpose of calculating the isocyanate index        refers to the total of active hydrogen atoms in hydroxyl and        amine groups present in the reactive compositions; this means        that for the purpose of calculating the isocyanate index at the        actual polymerisation process one hydroxyl group is considered        to comprise one reactive hydrogen, one primary amine group is        considered to comprise one reactive hydrogen and one water        molecule is considered to comprise two active hydrogens.

    -   3) Reaction system: a combination of components wherein the        polyisocyanates are kept in one or more containers separate from        the isocyanate-reactive components.

    -   4) The expression “elastomeric polyurethane material or foam” as        used herein refers to products as obtained by reacting        polyisocyanates with isocyanate-reactive hydrogen containing        compounds, using foaming agents, and in particular includes        cellular products obtained with water as reactive foaming agent        (involving a reaction of water with isocyanate groups yielding        urea linkages and carbon dioxide and producing polyurea-urethane        foams).

    -   5) The term “average nominal hydroxyl functionality” is used        herein to indicate the number average functionality (number of        hydroxyl groups per molecule) of the polyol or polyol        composition on the assumption that this is the number average        functionality (number of active hydrogen atoms per molecule) of        the initiator(s) used in their preparation although in practice        it will often be somewhat less because of some terminal        unsaturation.

    -   6) The word “average” refers to number average unless indicated        otherwise.

    -   7) “Density” is measured according to DIN 53420 and is moulded        density unless specified otherwise.

    -   8) Unsaturation in polyols is measured according to ASTM        D4671-05.

    -   9) “Polyurethane foam having a ball shape” refers to a ball        which consists entirely of polyurethane foam, which preferably        is one piece of foam.

The elastomeric polyurethane foam is prepared by reacting apolyisocyanate, which preferably is selected from the aromaticpolyisocyanates, and the selected polyol(s) and using a blowing agent.

The polyisocyanates preferably are selected from aromaticpolyisocyanates like toluene diisocyanate, naphthalenediisocyanate, andpreferably diphenylmethane diisocyanate (MDI), mixtures of MDI withhomologues thereof having an isocyanate functionality of 3 or more,which mixtures are widely known as crude or polymeric MDI, andisocyanate-terminated variants of these polyisocyanates, such variantscontaining urethane, uretonimine, carbodiimide, urea, allophanate and/orbiuret groups. Mixtures of these polyisocyanates may be used as well.

Most preferably the polyisocyanate is selected from 1) a diphenylmethanediisocyanate comprising at least 40%, preferably at least 60% and mostpreferably at least 85% by weight of 4,4′-diphenylmethane diisocyanateand the following preferred variants of such diphenylmethanediisocyanate: 2) a carbodiimide and/or uretonimine modified variant ofpolyisocyanate 1), the variant having an NCO value of 20% by weight ormore; 3) a urethane modified variant of polyisocyanate 1), the varianthaving an NCO value of 20% by weight or more and being the reactionproduct of an excess of polyisocyanate 1) and of a polyol having anaverage nominal hydroxyl functionality of 2-4 and an average molecularweight of less than 1000; 4) a prepolymer having an NCO value of 10% byweight or more and preferably of 15% by weight or more and which is thereaction product of an excess of any of the aforementionedpolyisocyanates 1-3) and of a polyol having an average nominalfunctionality of 2-6, an average molecular weight of 1000-12000 andpreferably an hydroxyl value of 15 to 60 mg KOH/g, and 5) mixtures ofany of the aforementioned polyisocyanates.

Polyisocyanate 1) comprises at least 40% by weight of 4,4′-MDI. Suchpolyisocyanates are known in the art and include pure 4,4′-MDI andisomeric mixtures of 4,4′-MDI and up to 60% by weight of 2,4′-MDI and2,2′-MDI. It is to be noted that the amount of 2,2′-MDI in the isomericmixtures is rather at an impurity level and in general will not exceed2% by weight, the remainder being 2,4′-MDI and 4,4′-MDI. Polyisocyanatesas these are known in the art and commercially available; for exampleSUPRASEC® MPR ex Huntsman.

The carbodiimide and/or uretonimine modified variants of the abovepolyisocyanate 1) are also known in the art and commercially available;e.g. SUPRASEC 2020, ex Huntsman Polyurethanes.

Urethane modified variants of the above polyisocyanate 1) are also knownin the art, see e.g. The ICI Polyurethanes Book by G. Woods 1990, 2^(nd)edition, pages 32-35.

Aforementioned prepolymers of polyisocyanate 1) having an NCO value of10% by weight or more are also known in the art. Preferably the polyolused for making these prepolymers is selected from polyester polyols andpolyether polyols.

Mixtures of the aforementioned polyisocyanates may be used as well, seee.g. The ICI Polyurethanes Book by G. Woods 1990, 2^(nd) edition, pages32-35. An example of such a commercially available polyisocyanate isSuprasec 2021 ex Huntsman Polyurethanes.

The polyols used are polyols comprising at least 30% by weight andpreferably at least 40% by weight (all calculated on the weight of thepolyol) of the selected polyol(s) having a level of unsaturation of atmost 0.03 meq/g; preferably this level is at most 0.01 meq/g. Theremaining at most 70% by weight and preferably at most 60% by weight ofthe polyol may be selected from other polyols like from polyols having alevel of unsaturation which is higher than 0.03 meq/g. The polyols(those having a low level of unsaturation as well as those having a highlevel of unsaturation) preferably have an average nominal functionalityof 2-4 and an average molecular weight of 1000-8000 and preferably of1000-7000.

The selected polyols having a level of unsaturation of at most 0.03meq/g are all known as such.

The selected polyester polyols may be selected from polycaprolactones,polycarbonate polyols and preferably condensation products ofpolycarboxylic and preferably dicarboxylic acids like adipic acid,glutaric acid, succinic acid and mixtures thereof and their anhydridesand mixtures thereof and glycols like ethylene glycol, diethyleneglycol, propylene glycol, dipropylene glycol, 1,4-butanediol and1,6-hexanediol and mixtures thereof, optionally with cross-linkers likeglycerol, trimethylolpropane and pentaerythritol and mixtures thereof.Such polyesters are widely known in the art. DALTOREZ® P708 is asuitable polyester polyol from Huntsman.

Polyxoypropylene polyols and polyoxyethylene polyoxypropylene polyolshaving such a low unsaturation are known as well. They may be producedby propoxylating an initiator using a catalyst like CsOH or a so-calleddouble-metal-cyanide catalyst and in the latter case by ethoxylationalong the propoxylation. Such polyols are being sold by Bayer asAcclaim™ polyols.

Polyoxyethylene polyols are also widely known. Examples are polyethyleneglycols having a molecular weight of 1000 to 8000.

Polyoxybutylene polyols and polyoxyalkylene polyols comprisingoxybutylene groups together with oxyethylene and/or oxypropylene groupsare also known. A commercially available example of a polyoxybutylenepolyol is POLYMEG ex Lyondell.

Polyoxyalkylene polyester polyols are also commercially available. Anexample is CAPA 7201A from Solvay.

The more preferred polyols having a level of unsaturation of at most0.03 meq/g are polyester polyols, polyoxybutylene polyols andpolyoxyalkylene polyester polyols and mixtures of these polyols andmixtures of the aforementioned polyols with polyoxyethylenepolyoxypropylene polyols having an oxyethylene content of 50-90% byweight, calculated on the weight of the polyol, in a weight ratio of theaforementioned polyols:this polyoxyethylene polyoxypropylene polyol of1:99 to 99:1 and preferably of 10:90 to 90:10; these polyols when havinga level of unsaturation of at most 0.01 meq/g, an average nominalfunctionality of 2-4 and an average molecular weight of 1000-7000 aremost preferred.

Polyols having a level of unsaturation which is more than 0.03 meq/g arewidely known as such. Examples are DALTOCEL® F428 and F435 ex Huntsman.

DALTOCEL, DALTOREZ and SUPRASEC are trademarks of Huntsman Corporationor an Affiliate thereof which have been registered in at least one butnot all countries.

A tennis ball according to the invention is a tennis ball whichcomprises an elastomeric polyurethane foam having a ball shape and adensity of 250-800 kg/m³, which foam is covered with a textile material,the foam having been prepared by reacting an aromatic polyisocyanate anda polyol comprising at least 30% by weight (on the weight of the polyol)of a polyol having a level of unsaturation of at most 0.03 meq/g whereinthis polyol having a level of unsaturation of at most 0.03 meq/g isselected from polyester polyols, polyoxypropylene polyols,polyoxyethylene polyoxypropylene polyols, polyoxyethylene polyols,polyoxybutylene polyols, polyoxyalkylene polyols comprising oxybutylenegroups together with oxyethylene and/or oxypropylene groups,polyoxyalkylene polyester polyols and mixtures of such polyols with theproviso that the use of a polyoxyethylene polyoxypropylene polyol havingan oxyethylene content of 50-90% by weight, calculated on the weight ofthe polyol, as the only polyol having a level of unsaturation of at most0.03 meq/g is excluded, and using a blowing agent.

In making the elastomeric foam a blowing agent is to be used. Theblowing agent should be used in such an amount that a density of 250-800kg/m³ is obtained. This amount may vary depending upon the type ofblowing agent used. Those skilled in the art will be able to determinethe amount in the light of the present description and the blowing agentchosen. Blowing agents may be chosen from physical blowing agents, likeCFC's and HCFC's and chemical blowing agents like diazodicarbonamide andwater. Mixtures of blowing agents may be used as well. Water is mostpreferred and preferably is used in an amount of 0.1-1.0% by weightcalculated on the amount of polyol.

In preparing the elastomeric polyurethane foam preferably anisocyanate-reactive chain extender and a catalyst are used.

The isocyanate-reactive chain extenders may be selected from amines,amino-alcohols and polyols; preferably polyols are used. Further thechain extenders may be aromatic, cycloaliphatic, araliphatic andaliphatic; preferably aliphatic ones are used. The chain extenders havea molecular weight of less than 1000 and preferably of 62-800. Mostpreferred are aliphatic diols having a molecular weight of 62-800, suchas ethylene glycol, 1,3-propanediol, 2-methyl-1,3-propanediol,1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-propanediol,1,3-butanediol, 2,3-butanediol, 1,3-pentanediol, 1,2-hexanediol,3-methylpentane-1,5-diol, 2,2-dimethyl-1,3-propanediol, diethyleneglycol, dipropylene glycol and tripropylene glycol, propoxylated and/orethoxylated products thereof and mixtures of these chain extenders. Theamount of chain extenders, if used, is 1-20% by weight calculated on theamount of polyol having a molecular weight of 1000 or more.

The catalysts used are catalysts enhancing the formation of urethanebonds like tin catalysts like tin octoate and dibutyltindilaurate,tertiary amine catalysts like triethylenediamine, imidazoles likedimethylimidazole, esters like maleate esters and acetate esters, andalkali metal or alkaline earth metal carboxylate salts like potassiumand sodium salts, especially the potassium salts. Examples are potassiumacetate, hexanoate, 2-ethylhexanoate and octanoate. If desired mixturesof catalysts may be used. The amount of catalyst will usually be in therange of 0.01 to 5, preferably 0.02-3 parts by weight per 100 parts byweight of reactants.

In addition to the above ingredients, additives and auxiliaries commonlyused in making elastomers may be used as optional ingredients; examplesare cross-linkers (i.e. isocyanate-reactive compounds having an averagenominal functionality of 3-8 and an average molecular weight of lessthan 1000 and preferably of less than 800), surfactants, fireretardants, smoke suppresessants, UV-stabilizers, colorants, microbialinhibitors, fillers, internal mould release agents and external mouldrelease agents.

The reaction to prepare the foams is conducted at an NCO index of 80-120and preferably of 90-110 and most preferably of 94-106.

The elastomers may be made according to the one-shot process, thesemi-prepolymer process or the prepolymer process.

The moulding process may be conducted according to the reactioninjection moulding process, the cast moulding process, rotationalmoulding and other known moulding processes.

The ingredients may be fed into the mould independently. Alternativelyone or more of the ingredients, except the polyisocyanate, are premixedand subsequently fed into the mould. In-line blending and impingementmixing may be used in the preparation process. Once the ingredients havebeen combined and mixed and fed into the mould they are allowed toreact. The temperature of the ingredients and of the mould may vary fromambient temperature to 100° C. The reaction time may be varied betweenwide ranges e.g. from 1 minute to 20 hours and preferably from 2 minutesto 10 hours; afterwards the elastomer may be demoulded. Any type ofmould may be used like metal moulds, silicon resin moulds and epoxyresin moulds. The over-pack applied in the process may vary from 120 to500%; over-pack being defined as the moulded density times 100%, dividedby the free rise density.

After demoulding the elastomer obtained preferably is post-cured.Post-curing may vary between wide ranges like between ½ hour and 6months and at a temperature between room temperature and 100° C. Thehigher the temperature the shorter the post-cure time.

Subsequently the elastomer is covered with textile material. Any textilematerial may be used; it may be woven and/or non-woven; and syntheticand/or not synthetic. Preferably this is the textile material usuallyemployed for making tennis balls e.g. a mixture of wool and syntheticfibre, e.g. nylon. Useful textile materials are Melton textile materialand Needle textile material, which are commercially available, and otherfelt-like materials. The colour of the textile material may be anycolour. Preferably the textile material has the colour usually employed;i.e. white or yellow. The textile material may be applied in any way.Preferably it is applied in the usual way; i.e. by adhering two dumbbellshaped blanks which on their reverse sides are coated with an adhesiveonto the surface of the elastomeric ball. The two dumbbell shaped blankspreferably have the same shape and the same size; together the size ofthese two blanks is about equal to the surface area of the ball. Theadhering of the two pieces to the ball may be conducted by means of anadhesive. Any suitable adhesive may be used. The two blanks preferablyare connected to each other preferably in a stichless way, e.g. bysticking the edges of the blanks to each other by means of an adhesive.

The invention is illustrated with the following example.

EXAMPLE 1

A polyol mixture was made by combining and mixing 54 parts by weight(pbw) of POLYMEG® 2000 polyol (BO polyol from Lyondell), 36 pbw ofSpecflex™ NC 700 (grafted polyether polyol from The Dow ChemicalCompany), 3.5 pbw of 1,2-ethanediol, 0.7 pbw of Dabco EG (catalyst fromAir Products), 0.02 pbw Fomrez® UL-1 (catalyst from Momentive), 0.5 pbwDabco DC-193 (surfactant from Air Products) and 0.5 pbw of water.

This polyol mixture and Suprasec 2733 polyisocyanate ex Huntsman werefed into a mould via a mixing head at index 100. The mould was analuminium mould consisting of 2 parts each having a hemisphere cavity.Both cavities were sprayed with external mould release agent, Acmosil36-4536. When the 2 parts are closed they together form a sphericalcavity having a diameter of 6.25 cm. The 2 parts are held together bymeans of clamping forces. The mould temperature was 70° C. The amount ofingredients used was selected in such a way that the density of the ballwould be 400 kg/m³.

When the mixture had been given 10 minutes to react an elastomericpolyurethane foam was demoulded having a spherical form. The ball wassubsequently cured in an oven at 80° C. for 12 hours and then underambient conditions for 6 weeks.

Then 2 dumbbell shaped pieces (same shape and size) of felt (thickness0.25 cm) were adhered (using adhesive) onto the surface of the ball. Thesurface of the 2 pieces together was the same as the surface of theball. The felt and the adhesive both were materials traditionally usedin making tennis balls.

EXAMPLE 2

Example 1 was repeated but using 1) a prepolymer made fromuretonimine-modified 4,4′-MDI and Daltorez P708, the prepolymer havingan NCO value of 16% by weight, instead of Suprasec 2733, 2) 90 parts byweight of Daltorez P720 instead of Polymeg 2000 and Specflex NC700 and3) 6 parts by weight of 1,2-ethanediol. The index was 100. Daltorez P708and P720 are polyester polyols obtainable from Huntsman; Daltorez is atrademark of Huntsman International LLC.

EXAMPLE 3 (COMPARATIVE)

Example 1 was repeated but using 54 pbw Daltocel F428 instead of Polymeg2000. Daltocel F428 is a polyoxyethylene polyoxypropylene polyol havinga nominal hydroxy functionality of 3, a molecular weight of 6000 and anoxyethylene content of about 15% by weight (all tipped); Daltocel is atrademark of Huntsman International LLC and Daltocel F428 is obtainablefrom Huntsman.

EXAMPLE 4 (COMPARATIVE)

Using the same procedure as above, a polyol mixture was made bycombining and mixing 84.05 parts by weight (pbw) Daltocel F428(unsaturation level of more than 0.03 meq/g.), 14 pbw of 1,4-butanediol,1pbw of Dabco 25S (catalyst from Air Products), 0.6 pbw of Jeffcat™ZF-22 (catalyst from Huntsman) and 0.35 pbw of water.

This polyol mixture and Suprasec 2433 polyisocyanate ex Huntsman werereacted as in example 1.

Properties of the tennis balls 1 day after the adherence of the 2 piecesof felt were as follows:

Example 1 2 3 4 Density of the ball without 400 400 400 400 felt, kg/m³Weight of the tennis ball, g 57 57 57 57 Rebound, cm (measured as 140131 112 125 described before) CLD, cm forward 0.54 0.70 1.27 1.0(measured as return 0.86 1.07 1.61 1.2 described before)

1. Tennis ball which comprises an elastomeric polyurethane foam having aball shape and a density of 250-800 kg/m³, which foam is covered with atextile material and which foam has been prepared by reacting anaromatic polyisocyanate and a polyol comprising at least 30% by weightof a polyol having a level of unsaturation of at most 0.03 meq/g andusing a blowing agent, wherein the polyol having a level of unsaturationof at most 0.03 meq/g is selected from polyester polyols,polyoxypropylene polyols, polyoxyethylene polyoxypropylene polyols,polyoxyethylene polyols, polyoxybutylene polyols, polyoxyalkylenepolyols comprising oxybutylene groups together with oxyethylene and/oroxypropylene groups, polyoxyalkylene polyester polyols and mixtures ofsuch polyols with the proviso that the use of a polyoxyethylenepolyoxypropylene polyol having an oxyethylene content of 50-90% byweight, calculated on the weight of the polyol, as the only polyolhaving a level of unsaturation of at most 0.03 meq/g is excluded. 2.Tennis ball according to claim 1 wherein the polyol has an averagenominal functionality of 2-4 and an average molecular weight of1000-8000.
 3. Tennis ball according to claim 1 wherein the polyol isselected from polyester polyols, polyoxybutylene polyols,polyoxyalkylene polyester polyols and mixtures of these polyols andmixtures of the aforementioned polyols with polyoxyethylenepolyoxypropylene polyols having an oxyethylene content of 50-90% byweight, calculated on the weight of the polyol, in a weight ratio of theaforementioned polyols:this polyoxyethylene polyoxypropylene polyol of1:99 to 99:1.
 4. Process for preparing a tennis ball according to claim1 comprising making a ball-shaped elastomeric polyurethane foam byputting an aromatic polyisocyanate, a polyol comprising at least 30% byweight of a polyol having a level of unsaturation of at most 0.03 meq/gand a blowing agent into a ball-shaped mould and by allowing theseingredients to form the elastomeric polyurethane foam, removing the foamfrom the mould and covering the foam with a textile material, whereinthe polyol having a level of unsaturation of at most 0.03 meq/g isselected from polyester polyols, polyoxypropylene polyols,polyoxyethylene polyoxypropylene polyols, polyoxyethylene polyols,polyoxybutylene polyols, polyoxyalkylene polyols comprising oxybutylenegroups together with oxyethylene and/or oxypropylene groups,polyoxyalkylene polyester polyols and mixtures of such polyols with theproviso that the use of a polyoxyethylene polyoxypropylene polyol havingan oxyethylene content of 50-90% by weight, calculated on the weight ofthe polyol, as the only polyol having a level of unsaturation of at most0.03 meq/g is excluded.
 5. Process according to claim 4 wherein thepolyol has an average nominal functionality of 2-4 and an averagemolecular weight of 1000-8000.
 6. Process according to claim 4 whereinthe polyol is selected from polyester polyols, polyoxybutylene polyols,polyoxyalkylene polyester polyols and mixtures of these polyols andmixtures of the aforementioned polyols with polyoxyethylenepolyoxypropylene polyols having an oxyethylene content of 50-90% byweight, calculated on the weight of the polyol, in a weight ratio of theaforementioned polyols:this polyoxyethylene polyoxypropylene polyol of1:99 to 99:1.
 7. Tennis ball according to claim 2 wherein the polyol isselected from polyester polyols, polyoxybutylene polyols,polyoxyalkylene polyester polyols and mixtures of these polyols andmixtures of the aforementioned polyols with polyoxyethylenepolyoxypropylene polyols having an oxyethylene content of 50-90% byweight, calculated on the weight of the polyol, in a weight ratio of theaforementioned polyols:this polyoxyethylene polyoxypropylene polyol of1:99 to 99:1.
 8. Process for preparing a tennis ball according to claim2 comprising making a ball-shaped elastomeric polyurethane foam byputting an aromatic polyisocyanate, a polyol comprising at least 30% byweight of a polyol having a level of unsaturation of at most 0.03 meq/gand a blowing agent into a ball-shaped mould and by allowing theseingredients to form the elastomeric polyurethane foam, removing the foamfrom the mould and covering the foam with a textile material, whereinthe polyol having a level of unsaturation of at most 0.03 meq/g isselected from polyester polyols, polyoxypropylene polyols,polyoxyethylene polyoxypropylene polyols, polyoxyethylene polyols,polyoxybutylene polyols, polyoxyalkylene polyols comprising oxybutylenegroups together with oxyethylene and/or oxypropylene groups,polyoxyalkylene polyester polyols and mixtures of such polyols with theproviso that the use of a polyoxyethylene polyoxypropylene polyol havingan oxyethylene content of 50-90% by weight, calculated on the weight ofthe polyol, as the only polyol having a level of unsaturation of at most0.03 meq/g is excluded.
 9. Process for preparing a tennis ball accordingto claim 3 comprising making a ball-shaped elastomeric polyurethane foamby putting an aromatic polyisocyanate, a polyol comprising at least 30%by weight of a polyol having a level of unsaturation of at most 0.03meq/g and a blowing agent into a ball-shaped mould and by allowing theseingredients to form the elastomeric polyurethane foam, removing the foamfrom the mould and covering the foam with a textile material, whereinthe polyol having a level of unsaturation of at most 0.03 meq/g isselected from polyester polyols, polyoxypropylene polyols,polyoxyethylene polyoxypropylene polyols, polyoxyethylene polyols,polyoxybutylene polyols, polyoxyalkylene polyols comprising oxybutylenegroups together with oxyethylene and/or oxypropylene groups,polyoxyalkylene polyester polyols and mixtures of such polyols with theproviso that the use of a polyoxyethylene polyoxypropylene polyol havingan oxyethylene content of 50-90% by weight, calculated on the weight ofthe polyol, as the only polyol having a level of unsaturation of at most0.03 meq/g is excluded.
 10. Process according to claim 5 wherein thepolyol is selected from polyester polyols, polyoxybutylene polyols,polyoxyalkylene polyester polyols and mixtures of these polyols andmixtures of the aforementioned polyols with polyoxyethylenepolyoxypropylene polyols having an oxyethylene content of 50-90% byweight, calculated on the weight of the polyol, in a weight ratio of theaforementioned polyols:this polyoxyethylene polyoxypropylene polyol of1:99 to 99:1.